Rotating-hearth furnace for reduction of metallic oxides

ABSTRACT

A method for improving the direct reduction of metal oxides in a continuous furnace with a rotating hearth. The method deposits two or several layers with increasing metal oxide contents in each layer towards a top loading surface and decreases carbon contents towards the top loading surface, which carbon will be preheated to a temperature of the order of 200° C. while the metal oxides will be preheated to a temperature of 800° C. The furnace is provided with equipment producing at the loading surface triangular grooves and in first and second zones of the furnace matching equipment with double action.

TECHNICAL FIELD

The present invention relates to an improved method for the directreduction (generally producing what is described as DRI) of metallicoxides, particularly iron oxides, by coal making it possible to achievea higher productivity and a lower specific consumption of coal. It alsorelates to an installation for implementing the method and to themetallic sponge capable of being produced.

1. Disclosure of the Invention

The invention aims firstly to increase the reaction rates by a mixing ofthe charge, by an increase in the efficiency of the radiation from thefurnace and by an increase in the surface area for heat exchange withthe furnace atmosphere, and secondly to use the reducing capacities ofthe volatile constituents of the coal more efficiently by their forcedpassage into a layer of preheated metallic oxides. Together, theseoperating conditions should also lead to a considerable increase in theproduction capacity per unit surface area and to a reduction in theproduction of carbon dioxide discharged into the atmosphere per unitquantity of the reduced metallic oxides obtained.

The invention also has as its objective the discharge of less dust tothe gas treatment plants thanks to a control over the speed of thesegases while keeping the volume of the furnace to a minimum.

The invention also aims to produce a metallic sponge having, in bulk, abetter homogeneity in the degree of reduction compared with productsresulting from known techniques.

2. Background Art

The direct reduction of metallic oxides, particularly of ores but alsoof various recycled metallic oxides, has developed considerably inrecent years.

A method for producing an iron sponge with a low sulphur content isdescribed in the document EP-0 692 543-A1.

This document indicates that a non-preheated charge is placed on amobile hearth, the said charge consisting of superimposed layers offinely divided materials, at least one of the layers consisting mainlyof iron oxides and at least one other layer being formed by a mixture ofa solid carbonaceous reducing agent and a desulphurising agent. The ironoxides are mainly reduced by means of carbon monoxide from the coal, thevolatile constituents of the coal taking part only partially in thereduction of the iron oxides.

Similarly, in the production of iron sponges from pellets formed from amixture of iron oxides and coal, the iron oxides are reduced principallyby means of the carbon monoxide from the coal, the volatile constituentsof the coal taking part only partially in the reduction process.

A method is described in the document LU-60981-A (Société Anonyme desMinerais) for producing an iron sponge comprising the use of acontinuous rotating-hearth reactor with a displacement of the materialfrom the side to the centre, first supplied with coal alone and thenwith iron ore, in pellet form or broken up, preheated to the reactiontemperature. Fixed scrapers cause a movement of the coal towards thecentre at each rotation. After other scrapers have been used to producea fairly thorough mixing of the heavily coked carbonaceous source andthe preheated iron ore, the charge is discharged through a centralshaft. The distillation gases from the coal and the reaction gases arepartly or completely burnt by heating the solid materials moving in thereactor before being discharged with the entrained fine particlestowards a collector located above the reactor.

This method (which is not illustrated by any drawings) does not includeefficient means for the best possible use of the reducing properties ofthe volatile constituents of the coal, the latter being used mainly fortheir heating capacity by combustion outside the reaction field.Moreover, the formation of the charge by superimposed layers ofmaterials not mixed beforehand and the use of scrapers moving the wholeof the charge each time do not enable a high degree of uniformity in thecharge to be obtained, either in material or in temperature, nor does itlead to a high productivity.

A method is described in the document U.S. Pat. No. 5,567,224-A(Kundrat) for producing an iron sponge in a rotating-hearth furnaceusing an oxidising flame located just above the upper surface of thehearth. A mixture of metallic oxides and a carbonaceous reducing agentis placed on the hearth, the said mixture passing in front of theburners during the rotation of the hearth. A second layer of thereducing agent is then charged and the heating is continued. Noarrangement for stirring and/or mixing the material is provided for. Thedischarge takes place towards the periphery and is achieved using anArchimedean screw.

A method is described in the document U.S. Pat. No. 3,383,199-A(Schmidt) for producing an iron sponge by supplying the materials on aconveyor belt in several layers, without any indication of the means forcarrying out the mixing of the constituents of each of the differentlayers within the layer or the mixing of the different layers with eachother.

A method is described in the document U.S. Pat. No. 3,770,417-A (Kranz)for the production of an iron sponge at the same time as the productionof coke based on the use of a distillation chamber placed above a heatedand perforated mobile hearth. The coal, after distillation, is added toa charge of oxides in the form of a second layer in order to bring aboutthe reduction. Scrapers mounted in the roof of the reactor stir thematerial and may be used to discharge it towards a central exit.

A method is described in the document U.S. Pat. No. 3,475,286-A(Kemmerer) for producing an iron sponge based on the use of a rotaryfurnace with a conical hearth provided with fixed scrapers mounted inthe roof of the reactor, which move the material towards the centre.Provision is made for arranging the scrapers in such a way that thethickness of the bed of materials on the hearth is kept constant.

The document “The Comet Process—DRI from fines and coal” published inSteel Times, vol. 224, no 11, November 1996, page 399 describes theprinciple of a rotating-hearth furnace developed by the Applicant incollaboration with the Center for Metallurgical Research in Liege(Belgium).

All the methods described in these documents have the same disadvantagesas those mentioned in connection with the above patent LU-60981-A, i.e.the volatile constituents of the coal participate only partially in thereduction of the metallic oxides and the methods do not make it possibleto obtain a high productivity or a high degree of uniformity as regardsthe temperature and the material of the charge.

Characteristic Elements of the Invention

The invention relies on the observation that, in most of the methods ofthe present state of the art, a major part of the volatile constituentsof the coal, particularly the hydrogen and the methane, are not used fortheir reducing capacity, this reduction being mainly carried out bycarbon monoxide, whose reduction kinetics are significantly lessfavorable than those of hydrogen. It therefore seems that it would beparticularly advantageous for the aforesaid volatile constituents to beprogressively released and, by their forced passage through a layercontaining the metallic oxides, for them to be put into contact with themetallic oxides under operating conditions (in particular, as regardsthe temperature of the metallic oxides and the successive mixtures ofreagents) such that they participate in the reduction of the latter.This implies that the metallic oxide and the released reducing gases areput into contact at temperatures as high as possible, but withoutupsetting the progress of the reduction process. To achieve this, thecoal will be preheated to a temperature of the order of 200° C., whilethe metallic oxides will be preheated to a temperature of the order of800° C., both constituents being preheated by means of heat recoveredfrom the discharged combustion gases using the same heat exchangers.

The invention concerns an improved method for the direct reduction ofmetallic oxides in a continuous rotating-hearth furnace, characterizedin that, on a part called the charging zone of the hearth over a widthof the ring, which depends on the diameter and the capacity of thefurnace, two or more layers are deposited which have concentrations ofmetallic oxides per layer that increase towards the upper surface of thecharge and concentrations of coal, particularly of coal with a highconcentration of volatile constituents, that decrease towards the uppersurface of the charge, said coal being preheated to a temperature of theorder of 200° C. while the metallic oxides are preheated to atemperature of the order of 800° C., both constituents being preheatedby means of heat recovered from the discharged combustion gases usingthe same heat exchangers, in order to take the coal rapidly to atemperature at which the volatile constituents are released and, underthe effect of the volatile constituents, particularly hydrogen, releasedin this way, in order to carry out the reduction of the preheatedmetallic oxides at a temperature sufficient to initiate the reductionreaction, the entrance to the furnace being provided with equipmentcreating triangular furrows in the surface of the charge, and the firstzone of the furnace being provided with additional dual-action equipmentmaking it possible, by a first action in which the peaks are levelledoff, to prevent the peaks of each furrow, which are very rapidly heated,reaching the temperature of agglomeration, which would make their mixingwith the charge more difficult, the levelled-off part being taken intothe hollows of the furrows and, by a second action, to achieve alevelling off of a face of each furrow as far as its base, thelevelled-off part being pushed on to a face of the adjacent furrow andcovering the material brought by the first action, and thanks to whichthe charge is progressively mixed at increasingly deep levels and ismoved radially as the hearth rotates, the base of the furrows beingmoved at the end of each revolution of the charge radially in one ormore stages through a total distance corresponding to the width of eachcharging zone, and the second zone of the furnace being provided withdual-action equipment making it possible, through a first action oflevelling off the peaks, to prevent the peaks of each furrow, which arevery rapidly heated, reaching the temperature of agglomeration, whichwould make the mixing more difficult through the effect of segregation,the levelled-off part being pushed into the hollows of the furrows, andthrough a second action to level off a face of each furrow down to thehearth, the levelled-off part being pushed on to a face of the adjacentfurrow and covering the material brought by the first action, the chargebeing moved radially as the hearth rotates to be discharged afterseveral revolutions, preferably after 4 or more revolutions, towards thepart of the ring opposite the charging zone.

In a particular application, the layer of the mixture of metallic oxidesand coal and the layer of metallic oxides consists of a layer of pelletsincluding these constituents.

The following description will refer to the general term “metallicoxides”. This term embraces the usual metallic ores, recycled metallicoxides originating from iron and steel-making processes, for examplefrom blast furnaces, steel plants, electric furnaces or rolling mills,as well as a mixture of these sources of oxides with coke fines or withcoal, if necessary in the form of pellets.

Coal is to be understood to mean any solid carbonaceous material.

In the first and second zones of the furnace, the operating conditionsare chosen in such a way as to achieve a compromise between, firstly,the need to produce a high and uniform temperature of the charge asquickly as possible and, secondly, the need to put progressively incontact with the layer of metallic oxides or the upper layer of themixture of metallic oxides and coal only the upper part of theunderlying layer of coal, avoiding incorporating in it the cooler lowerlayers, so that the temperature of the new mixture thus obtained isabove 600° C., in particular of the order of 700° C.

The rotational speed of the hearth may lie between 3 and 12 revolutionsper hour. It is preferably 8 revolutions per hour.

Moreover, as regards the upper layer of the charge, it is essential toavoid this being vitrified, for example by the formation of silicates ofthe fayalite type that have an inhibiting effect on the reduction. Forthis purpose, means such as rabbles ensure a rapid mixing of the surfacelayer in the layer immediately below.

The final aim is of course to obtain, in a production time as short aspossible, for a thickness of charge of the order of 5 to 10 cm, theproduction time being determined by the coldest point in the charge, ametallic sponge with a better homogeneity than the sponges produced byreduction methods of the present state of the art, the latter generallyhaving the drawback that they yield a product in which the metallicoxides are reduced by varying degrees.

It has been found advantageous to mix some lime with the metallicoxides, firstly because said lime acts as a catalyst for the reactionand secondly because it prevents phenomena of adhesion in the metallicsponge. In addition, the lime generally contributes to desulphurisationof the pig iron and to the formation of a more fluid slag or clinker.

Description of a Preferred form of Execution of the Invention

According to a preferred form of execution, provision may be made forthe following:

the charging is carried out in the inner contour of the ring, the smallcircle, preferably over ⅛ to {fraction (1/12)} of the width of the ring,

the material which undergoes 4 or more complete revolutions, dependingon the charging conditions over the width of the ring, is turned over upto 100 times by rabbles provided with blades of different shapes andfunctions depending on the zone of the furnace as described above,

at each blade, the charge is moved radially outwards, the charge thusdescribing a roughly helical path,

discharging is carried out over the outer part of the ring by means ofone or two worm conveyors having a length corresponding respectively tothe width or to half the width of the charging,

burners are placed on the walls at the sides of the ring, mainly on theouter walls of the ring, on the large circle,

the gases are discharged by flowing in a direction opposite that of themovement of the materials through the walls on the inner sides of thering, on the small circle.

On the rabbles, the dual-action blades with different dimensions andshapes are so arranged that the blades in the first zone progressivelylevel off the charge at increasingly deep levels down to the base of thefurrows, while the blades in the second zone, where the charge is stillnot agglomerated and is still easily mixable, have an appropriate shapedifferent from that of the first blades and they level off the furrowsand their base. This prevents the appearance, on the surface of thecharge, of a sheet of reduced metallic oxides that is too thick, strong,difficult to break up and difficult to discharge.

The advantages are mainly a more uniform reduction of the charge, asimpler charging installation, easier discharge and a more effectivecontrol over the furnace atmosphere by a better control of the gases inthe furnace.

The rabbles are fixed and are placed radially in the furnace, the firstrabble being located in the charging zone, i.e. the zone in which thefurnace is supplied with material.

The blades of the rabbles are fixed and offset, i.e. arranged in aslightly staggered fashion with respect to the furrows formed by theblades of the preceding rabble, by 50 mm for example, so as to level offa sloping side of each furrow. The movement of the material on thehearth causes mixing (i.e. stirring) and the formation of a new furrow.The blades create triangular furrows over the whole surface of thecharge and this increases the surface area of the charge at theinterface with the furnace atmosphere by about 35%, thus producing agreater heat transfer from the furnace to the charge.

The first and second types of dual-action blades are designed so that,at each passage through the charge, a part of it is turned over, theupper layer of the charge in contact with the furnace atmosphere,initially consisting of metallic oxides, and then of the mixture ofmetallic oxides and coal and finally of reduced metallic oxides,descending while the underlying layer is raised.

The end of each of the blades is shaped in such a way as to turn thematerial over so that the topmost part of the furrow, the hottest part,is moved to the trough of the newly created furrow, in order to ensurebetter homogenization.

Said end may, if desired, be cooled (by the internal circulation of acooling liquid, for example).

The rabbles may be distributed linearly in the different zones of thefurnace over the length of the passage in a zone of the furnace. It willpreferably be made non-linearly and will be dependent on the surfacetemperature and on the temperature gradient in the charge.

The amount of coal is determined by the stoichiometric quantitynecessary to bring about the complete reduction of the metallic oxidespresent, reduced by an amount corresponding to the reducing action ofthe volatile elements, and possibly increased by an amount necessary formelting the sponge and for subsequent alloying.

The progressive mixing of the layer of metallic oxides with theunderlying layer, whose temperature is necessarily higher in the zonenear to the interface between the metallic oxides and the coal than inthe more distant layers, has the following consequences:

a greater heat transfer through an increase in the surface area at theinterface between the upper layer and the furnace atmosphere;

the higher thermal conductivity of the layer of metallic oxides,initially present in a single layer at the upper part of the charge andafterwards progressively in the mixture, contributes to a better heattransfer than that in methods with multiple layers, without the reducingagent, in this case coal, which is a poorer conductor of heat,disturbing the process;

the progressive mixing of the layers forming the charge enables auniformity of the temperature throughout the charge to be rapidlyachieved;

the metallic oxides very rapidly reach the high temperatures where theirreactivity is greater, which increases the efficiency of the reductionprocess and reduces the operational time;

the volatile constituents released progressively and generated by thecoal taken progressively to higher temperatures are used efficiently anddirectly as a reducing agent;

the reduction using hydrogen occurs immediately and is optimised, whichenables advantage to be taken of the fact that its reaction kinetics arebetter than those of CO gas;

the reduction by CO is rendered more efficient because the hotter upperlayer is progressively mixed with the layer immediately below taken toan adequate temperature and not with the deeper layers that are stilltoo cool;

in principle, it becomes possible to produce less carbon dioxide perunit mass of the reduced metal produced;

surface temperatures that are too high are avoided and hence there is noproduction of fayalite;

the appearance on the surface of the charge of a sheet of reducedmetallic oxides that is too thick, strong, difficult to break up anddifficult to discharge is prevented;

the furnace, for a given production, will be less bulky than that inother methods, for example in those working with a multi-layered chargeor a charge consisting of a single layer of pellets, for example.

In principle, the reducing agent is coal with a high concentration ofvolatile constituents, preferably with a concentration of volatileconstituents greater than 25%.

The furnace is generally maintained at a dome temperature of the orderof 1300 to 1450° C., preferably of the order of 1400° C., by burnersinstalled in the outer walls of the mobile-hearth furnace and withpost-combustion in the inner part of the ring.

The successive mixing of the upper layers with the underlying layersmeans that the maximum surface temperature reached does not exceed 1100to 1200° C.

The methods used also make it possible to increase the homogenisation ofcharges consisting of pellets, which contributes to a considerableincrease in the thickness of the charge, to a faster and more efficientoperational cycle, to a more compact furnace and to an optimisation ofheat exchanges.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be described with reference to a preferred form ofexecution of the invention illustrated in the appended drawings.

FIG. 1 shows a diagrammatic horizontal projection of a rotary furnacewith a distribution of rabbles in a rotary furnace.

FIG. 2 shows a vertical projection of a section through the rotaryfurnace.

FIG. 3 shows the furrows formed during the charging.

FIG. 4 shows the furrows in the charge resulting from the first actionof the blades, located on fixed rabbles.

FIG. 5 shows the furrows in the charge resulting from the second actionof the blades, located on fixed rabbles.

FIG. 6 shows a diagrammatic view of a vertical projection of a sectionthrough a rabble and a blade with an arm fixing it to the rabble.

DESCRIPTION OF A PREFERRED FORM OF EXECUTION OF THE INVENTION

The operational principle of the method is illustrated in FIG. 1.

In FIG. 1, the charging zone is illustrated at 1 and the zone fordischarge from the rotating hearth 3 is illustrated at 2, said hearthexecuting a movement in the counter-clockwise direction represented bythe arrow 4 about the furnace axis 5. The burners fixed in the outerwall of the furnace are represented at 6, the combustion gases areextracted through the inner walls of the furnace at 7 and are sent toheat exchangers through openings 8. The rabbles supporting the bladesare referred to by the reference number 9, while the oxygen injectorsare referred to by the reference number 10.

The same reference numbers as in FIG. 1 are used in FIG. 2. Thereference number 11 denotes the charge.

FIG. 3 shows the furrows 12 formed during the charging.

FIG. 4 shows the levelling off of the peaks 13 of the furrows resultingfrom the first action of the blades.

FIG. 5 shows the levelling off 14 of the furrows resulting from thesecond action of the blades.

FIG. 6 shows a diagrammatic view of a vertical projection of a sectionthrough a rabble 15 with its external thermal insulation 16 and an innerwater-cooled chamber 17 together with a blade 18 having an arm 19 fixingit to the rabble.

What is claimed is:
 1. Rotating-hearth furnace for the reduction ofmetallic oxides comprising: a ring-shaped rotating hearth subdividedinto: a charging zone, a first zone adjacent to the charging zone, asecond zone adjacent to the first zone and a discharge zone adjacent tothe second zone, the charging zone comprising a device for depositing acharge with one or more layer of a mixture of metallic oxides and coaland equipment for forming on a surface of the deposited layer furrowshaving a substantially triangular section so as to obtain asubstantially sawtooth-shaped surface, the first zone of the furnace andthe second zone comprising blade equipment for leveling off the peaks ofthe sawteeth, a levelled-off part of the peaks being pushed into thehollows of the furrows and, by a second action, to level off one face ofeach saw tooth, the levelled-off part being pushed on to a face of theadjacent sawtooth so as to cover the material pushed by the firstaction, and displacing the mixture radially as the hearth rotates, saidequipment of the first zone and second zone comprising respectively,first and second dual action blades such that upon each passage throughthe charge, a part of the charge is turned over, such that an upperlayer of the charge initially comprises metallic oxides, then a mixtureof metallic oxides and coal, and finally comprises reduced metallicoxides wherein the successive mixing of the parts of the charge limitthe maximum surface temperature thereof to a temperature less than orequal to 1200° C.
 2. The furnace according to claim 1, which comprisesone of a worm conveyor and a deflector for carrying a discharge out overa part of the ring.
 3. The furnace according to claim 1, which comprisesmeans for progressively stirring upper layers and underlying layers ofthe charge so as to successfully mix the upper layers of the charge withthe underlying layers.
 4. The furnace according to claim 1, wherein saidfirst and second dual-action blades comprise teeth portions of first andsecond ravels, respectively, said ravels being fixed and arrangedradially in the furnace.
 5. The furnace according to claim 1, whereinthe ravels located in the first zone comprise blades penetrating thelayer of the charge up to a base of the sawteeth for displacing themixer radially towards the discharge side of the ring.
 6. The furnaceaccording to claim 1, wherein the ravels located in the second zonecomprise blades for penetrating the layer over an entire depth portionthereof, and for displacing the mixture radially towards the dischargeside of the ring.
 7. The furnace according to claim 1, wherein theblades are offset from one another by being arranged in a slightstaggered fashion with respect to the furrows worn by the blades so asto level off one sloping side of each furrow and thus form a new furrow.8. The furnace according to claim 1, wherein the blades are adapted tocontinuously turn the charge over until discharge thereof from thefurnace.
 9. The furnace according to claim 1, which comprises aplurality of burners positioned on an outer wall portion of the furnacefor maintaining a temperature in the furnace so as to be in the range of1300 to 1450° C. at a dome portion of the furnace.